1. Field of Invention
The present invention relates to an optical element for converting light incident thereto to a predetermined polarized light beam, as well as a polarization illumination device and a projection display apparatus using the same.
2. Description of Related Art
In order to increase the efficiency with which light is utilized to provide a bright display, illuminating optical systems of projection display apparatuses make use of a method that converts polarized light with random polarization directions to polarized light with one polarization direction. A known optical element (polarization conversion element) for converting light with random polarization directions to light with one polarization direction is disclosed in Japanese Patent Laid-Open No. 7-294906. FIGS. 14A and 14B are plan view of such an optical element containing a polarization beam splitter array 20 formed by alternately bonding together linear polarization beam splitters 30 including polarization separating films 36 and linear prisms 40 including reflecting films 46. A lens array composed of a plurality of focus lenses is provided at the light-incoming surface of the polarization beam splitter array 20, with ½ phase plates 24 selectively provided at the light-outgoing surface of the splitter array 20.
As shown in FIG. 14A, light incident upon the lens array 10 is gathered and converted by the lens array 10 into a plurality of separated light beams (or intermediate light beams) that are incident upon the polarization beam splitters 30 disposed in correspondence with the lens array 10 in the form of light beams containing s-polarized components and p-polarized components. The incident light is separated into s-polarized light beams and p-polarized light beams by the polarization separating films 36. Each s-polarized light beam is reflected substantially vertically by its corresponding polarization separating film 36 formed at an angle of 45 degrees with respect to the light-incoming plane. Then, each s-polarized light beam is reflected again by its corresponding reflecting film 46 formed at an angle of 45 degrees with respect to the light-incoming plane, and leaves its associated prism 40. On the other hand, each p-polarized light beam passes through its corresponding polarization separating film 36, and is converted into an s-polarized light beam by its associated ½ phase plate and leaves it. Thus, this optical element is of the type that converts all of the incident light beams with random polarization directions to be converted into s-polarization light beams before leaving the optical element.
Ideally, all of the light incident upon the lens array 10 and gathered by each of the focus lenses of the lens array 10 should be incident upon the polarization beam splitters disposed in correspondence with each of the focus lenses. As shown in FIG. 14B, however, there are actually portions of the light incident upon the lens array 10 that are not completely gathered by the array 10, causing them to be incident upon their corresponding prisms 40. The light portions incident upon the prisms 40 are totally reflected at their corresponding reflecting film 46, and strike the polarization beam splitters 30 adjacent thereto. The light incident upon the polarization beam 30 is separated into s-polarized light beams and p-polarized light beams by the polarization separating films 36. The s-polarized light beams produced by the separation are reflected by their respective polarization separating film 36, and are converted into p-polarized light beams by their respective ½ phase plates and leave them. On the other hand, the p-polarized light beams pass through their respective polarization separating films 36, and are reflected by and leave the reflecting films 46 disposed in the direction of passage of the light beams. Thus, some of the light incident upon the optical element (polarization conversion element) leaves the optical element as s-polarized light beams, and some of the light incident upon the optical element leaves the optical element as p-polarized light beams. Here, the light-incoming surface of the polarization conversion element can be divided into effective light-incoming areas EA and ineffective light-incoming areas UA. The effective light-incoming areas EA are areas of the light-incoming surface of the polarization conversion element where the incident light beams are converted into the desired polarized light beams and leave the polarization conversion element. On the other hand, the ineffective light-incoming areas UA are areas of the light-incoming surface of the polarization conversion element where the incident light beams leave the polarization conversion element after being converted into undesired polarized light beams. 
When there is a desire to use only one type of polarized light beam, it is necessary to cut off the light beams traveling toward the ineffective light-incoming areas UA using a polarization plate or the like. In this case, the aforementioned outgoing p-polarized light beams are not used, thereby reducing the efficiency with which light is used.